A light emitting diode (LED) is principally formed by multiple epitaxial layers of a light emitting semiconductor material. For example, a blue-light LED is mainly consisted of gallium nitride-based (GaN-based) epitaxial thin films that are stacked into a light emitting body in a sandwich structure. To effectively extract excited light generated by the light emitting body and at the same time enhance light emitting efficiency, LEDs are categorized into horizontal, vertical and flip-chip LEDs.
Referring to FIG. 1, a conventional horizontal LED 1 includes a reflecting layer 2, an N-type semiconductor layer 3, an N-type electrode 4, a light emitting layer 5, a P-type semiconductor layer 6, a current block layer 7, a transparent conductive layer 8, and a P-type electrode 9. The N-type electrode 4 and the P-type electrode 9 input a voltage difference 10, so as to drive the sandwich structure of the N-type semiconductor layer 3, the light emitting layer 5 and the P-type semiconductor layer 6 to generate excited light 11. The reflecting layer 2 reflects the excited light 11 such that the excited light 11 exits via a same side in a concentrated manner.
To prevent the opaque P-type electrode 9 from excessively shielding the excited light 11 and hence resulting low light extraction efficiency, the P-type electrode 9 is defined with a certain area. However, a current passing through the light emitting layer 5 may get too concentrated if the P-type electrode 9 is too small, in a way that light emitting uniformity and efficiency of the light emitting layer 5 may be unsatisfactory. Therefore, to maintain the light uniformity and efficiency of the light emitting layer 5 and to at the same time reduce the shielding area of the P-type electrode 9, the P-type electrode 9 needs to be applied in collaboration with the transparent conductive layer 8 that is both conductive and transparent. Alternatively, the transparent conductive layer 8 may be directly implemented as the P-type electrode 9. When a current is induced from the P-type electrode 9, the current is allowed to diffuse via the transparent conductive layer 8 to enhance the light emitting uniformity and efficiency of the light emitting layer 5.
Nonetheless, as most of the current takes a shortest route, i.e., travels from the P-type electrode 9 directly downwards to pass through the transparent conductive layer 8, the diffusion achieved by the transparent conductive layer 8 is quite limited. To promote the diffusion within the transparent conductive layer 8, the current block layer 7 is conventionally disposed between the transparent conductive layer 8 and the P-type semiconductor 8, at a a region of the P-type electrode 9. The current block layer 7 blocks the current from passing through, and so the current is forced to detour along the current block layer 7 to be diffused at the transparent conductive layer 8, thereby enhancing the light emitting uniformity and brightness of the light emitting layer 5.
The transparent conductive layer 8 is generally made of indium tin oxide (ITO). Although being a transparent material, ITO does not have a high transparency. That is, ITO tends to absorb light. For the above structure, when the transparent conductive layer 8 is made of ITO, the diffusion of the current can be promoted to enhance the light emitting uniformity and efficiency, with however a considerable amount of light loss being resulted meantime as well. More particularly, when the excited light 11 is reflected for a number of times in the LED 1 and hence passes through the transparent conductive layer 8 for a number of times, a significant amount of light loss is resulted.